Novak Djokovic wins the ATP World Tour final

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Serb seals second ATP World Tour Finals win and fourth straight title since September

Djokovic admits to doubts following difficult defeats to Nadal at French and U.S. Opens

26-year-old set to lead Serbia in Davis Cup final in Belgrade starting on November 15

A year that began with a fourth Australian Open title ended in similarly triumphant style for Novak Djokovic on Monday as the Serb outclassed Rafael Nadal in the ATP World Tour Finals finale.

The stunning straight-sets 6-3 6-4 victory over the world no.1 ensured the Serb finished the season with a flourish, claiming four titles in a row and complete a run of 22 matches unbeaten since losing to Nadal in the U.S. Open final in September.

"I've had a great 2013 season and this is the best possible way to crown all the achievements that I had," Djokovic told CNN's Amanda Davies.

"This has been the best tennis (I've played) in the whole of 2013. And winning against the world No.1 Nadal, who has had the best year out of all the players, in straight sets could not be better," Djokovic added.

"I didn't expect myself to play as well as I did in the last two-and-a-half months after that painful loss in the U.S. Open final."

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The four-set defeat to the Spaniard at Flushing Meadows and, in particular, the semi-final loss at the French Open last June, were hard to take says Djokovic.

"I played well in those matches but he just got the best out of me in the important moments," added the Serb.

"Mentally, I didn't keep my composure, my calmness. When you play a top rival, a few points decide a winner and it can shift from one side to the other in a split second," he said.

"I did have my doubts after that and had some ups and downs. But I managed to get stronger and had that necessary experience to understand what I need to get better, to get stronger and to have this willpower that keeps me going.

"I've managed to regroup to analyze what I've done wrong and work on that with my team and we had a great improvement in the last two months."

The recent run of victories ranks second only to the incredible 41-match winning streak he achieved in 2011, though Djokovic happily conceded Nadal is a worthy world No. 1 right now.

"I have to say he deserves to end up as No. 1 because he won two grand slams and has played incredible tennis on hard courts this season. (It's) not his preferred surface in the past few years, but he has improved and won so many matches," Djokovic said.

"For me, it's just important to keep on winning, to keep on getting as many titles as I can and I know the rankings and results will follow."

While Nadal's own remarkable season is at an end, Djokovic has one more mountain to climb as he attempts to lead Serbia to Davis Cup glory against Czech Republic in Belgrade this weekend.

The 26-year-old says that the international team tournament might lack the kudos it once enjoyed, but he's still desperate to add a second title to the one Serbia clinched against France in 2010 before gearing up for more individual glory next year.

"Representing your country is very rare in our sport and a very unique opportunity for us to win another title at home," he says.

"This can serve as a great platform for 2014 season. I'm extra motivated and inspired to work and get myself, first of all, rested and recovered in the off season, and then work on my game and get ready for Australian Open."